In known devices, a method known as optical coherence tomography (OCT) is increasingly being used for monitoring (laser) processing operations. This method is based on the fundamental principle of interference of light waves and the resulting effects. Optical coherence tomography makes it possible to detect differences in height along a measurement axis in the micrometer range. To do so measurement light is generated and split by means of a beam splitter into a measurement beam and a reference measurement beam. The measurement beam is conveyed further to a measurement arm where it strikes the surface of a measurement sample, for example, a workpiece to be machined. The measurement beam is at least partially reflected on the surface and directed back at the beam splitter. The reference measurement beam is sent further to the reference arm and is reflected at the end of the reference arm. The reflected reference measurement beam is also sent back to the beam splitter. Finally, the overlap in the reflected beams is detected to determine height information about the surface and/or the depth of penetration of a processing beam, taking into account the length of the reference arm.
Simple coupling of an OCT measurement device with a (laser) processing device is possible since the measurement beam of an optical coherence tomograph runs coaxially with a processing beam and may overlap it.
The documents WO 2012/037694 A2 and US 2012/0285936 A1 describe different embodiments of laser processing devices having interferometric measurement systems for monitoring the laser processing operation, for example, by measuring the depth of penetration of the laser beam.
For the most thorough possible monitoring of a processing operation, in addition to detecting the depth of penetration of the processing beam and the instantaneous processing position on the workpiece (in the area of the steam capillary and the weld pool) the ambient region of the processing position is also to be observed. In this regard, the document DE 101 55 203 A1 describes the fixed alignment of one measurement beam or multiple measurement beams with various regions on the workpiece. This fixed alignment of the respective measurement beam is achieved by a fixed orientation of the measurement beam with input into a beam splitter.
To implement scanning of an ambient region of the processing position on a workpiece, known (laser) processing devices and/or measurement devices are provided with various scanning devices which permit deflection of a measurement beam.
The document DE 10 2103 008 269 A1 describes a laser processing device having an optical coherence tomograph, which includes a scanning device having a plurality of galvanically suspended mirrors (galvanic mirrors). The scanning device is arranged in the beam path of the laser processing device in such a way that controlled adjustment of the galvanic mirror produces a defined shift of the measurement beam as well as the processing laser beams on the workpiece. The measurement beam here is always directed at an instantaneous processing position due to the joint displacement. For detection of the ambient area, in another embodiment, the OCT measurement device has a planar wobble mirror, which is mounted so that it can wobble and can therefore shift the measurement beam on the workpiece in a circular path defined by the wobble mirror around the instantaneous processing position of the processing beam.
A simple displacement of a measurement beam onto ambient regions of a processing position by means of a wobble mirror limits the scanning to a circular path on a workpiece. This has the disadvantage that a relatively large radius of the circular path must be provided to detect regions which are disproportionately a great distance in front of or behind a processing position with respect to a main processing path. Therefore, the measurement beam must be shifted on the workpiece by a comparatively great distance on an arc of a circle, for example, to go from an area behind the processing position to an area in front of the processing position. This reduces the efficiency of a scanning operation and also limits the number of measurement points in a certain measurement area, for example, in front of or behind the processing position.
An alternative to deflection of a measurement beam for scanning a workpiece by means of a wobble mirror is described in the document DE 10 2012 016 788 A1. This document discloses a deflection of a measurement beam by rotation of a polygonal mirror. The polygonal mirror can be rotated at a high rotational speed above an axis of rotation and can thereby deflect an incoming measurement beam in the desired manner. To permit a curved scanning path of the measurement beam on the workpiece, a rotating polygonal mirror having a shaped three-dimensional mirror surface is described.
However, the construction and fabrication of three-dimensional free-form surfaces on a polygon/polygonal body are complex and are associated with a high technical and financial outlay. Furthermore, with the arrangement described here the scanning path of the measurement beam on the workpiece is structurally determined by the shape of the free-form surfaces of the polygonal mirror and cannot be varied during the measurement operation.
The document EP 1 977 850 B1 describes a processing device with an OCT scanning device, which includes a deflecting mechanism and is provided for surface scanning of a workpiece. The deflecting mechanism has at least one movably suspended oscillating mirror, which is equipped to guide the measurement beam for scanning the workpiece surface over the workpiece surface. To focus the measurement beam again after being deflected to focus it by means of the oscillating mirror, the arrangement of an F-theta lens array between the oscillating mirror and the workpiece is also disclosed.
The closest prior art to the present invention is known from the document DE 10 2013 015 656 A1. This document describes a laser processing device comprising an optical coherence tomograph. To detect surface information about the workpiece and in particular the depth of penetration of the processing beam, two measurement beams are formed from one OCT measurement light for the purpose of performing a measurement. It is provided here that the first measurement beam is directed as accurately as possible into a steam capillary of the welding point, i.e., onto the prevailing processing position on the workpiece, and remains there. This serves to monitor the instantaneous depth of penetration of the processing beam into the workpiece. Furthermore, it is described that an adjustment can be carried out before starting the processing operation in order to ensure the accuracy of the measurement of the depth of penetration of the processing beam. To do so lenses can be tilted and/or shifted into the beam path of the measurement beams. The adjustment serves to define the position of the first measurement beam as accurately as possible with the respective instantaneous processing position.
In this state of the art, scanning of a first area around the processing position is implemented with the help of the second measurement beam, which is deflected for scanning on the workpiece by means of a rotary wedge plate, an oscillating mirror or a rotating aspherical optical element. These deflecting components are arranged in the beam path of the second measurement beam in the individual embodiments before the beam is input into the processing head of the device.
In displacement of the measurement beam for scanning ambient areas of the processing position, according to the state of the art, areas of the workpiece surface which are of little or no interest are always also scanned because they are not important for the processing operation that is to be monitored. This reduces scanning efficiency and thus also the efficiency of the monitoring process.